1. Field of the Invention
This invention relates in general to oil well tools and in particular to a device for activating an explosive downhole.
2. Description of the Related Art
Explosives are set off downhole in an oil or gas well for various purposes including, for example, perforating a casing, cutting a wire line or setting a packer. Numerous patents have addressed the subject of conveying an explosive safely downhole and detonating the explosive at a desired location.
Detonating devices have frequently been conveyed into a well bore on an electric wire line. An electric wire line offers the advantage of providing control of the tool from the surface. However, there are disadvantages to using an electric wire line, including the expense of setting up and deploying the wire line. Further, deviated or horizontal wells are being drilled more frequently now, and gravity cannot be relied upon to deploy a tool into a horizontal well. Coiled tubing may be required to push a tool into a desired position in a horizontal well bore.
Where a tool can be lowered into a well bore, the use of a non-electric or "slick" wire line is preferable in that it is more economical. U.S. Pat. No. 4,306,628, issued to Adams, Jr. et al. exemplifies the prior art for a mechanical switch that can be deployed on a slick wire line and yet be controlled from the surface. Adams, Jr. et al. disclose a mechanical safety switch for closing electrical contacts in a well tool. The mechanical switch has several safety requirements which must be satisfied before electrical contacts will close, including upward pull on a wire line and sensing a preselected value of fluid pressure in the well bore. The tool incorporates a motion sensor and a timer in a circuit, the circuit including a mechanical switch that is closed by upward pull on the wire line for activating an explosive device.
The Adams, Jr. et al. device has a trigger that extends laterally from the tool so as to contact the inside wall of a cased well bore. When the device is lowered to a proper depth, an operator at the surface pulls upwardly on the slick wire line. The trigger should catch on the inside wall of the casing when the tool is pulled upwardly so that the trigger is pulled into a second position, closing a set of electrical contacts and enabling a circuit.
Difficulties were experienced with this mechanical apparatus. The tool was subsequently modified to eliminate the trigger mechanism activated by upward pull on the wire line. Well pressure was instead used to close an electrical switch, with well pressure moving a piston longitudinally within the tool. Unfortunately, these modifications led to an incident where debris was trapped on the piston leaving the electrical switch in a closed position. When an operator at the surface reloaded an explosive charge, the switch was closed, activating the explosive and injuring the operator severely.
The American Petroleum Institute developed Recommended Practice 67 for handling oilfield explosives safely. Section 4.3 of Recommended Practice 67 states that detonators and initiators must incorporate at least one of the following features: a minimum DC resistance of 50 ohms with a minimum "no-fire" current of 200 milliamperes; a high voltage exploding bridgewire design; a high voltage exploding foil initiator design; or a system offering equivalent safety.
The high voltage exploding bridgewire design and the high voltage exploding foil initiator design require a high-voltage charge for activation. A tool conveyed downhole by a slick wire line or coiled tubing includes a self-contained power supply, typically a battery pack. Consequently, tools conveyed by slick wire line or coiled tubing typically do not use the high voltage exploding bridgewire or foil initiator design because a self-contained power supply typically cannot provide sufficiently high voltage. Tools conveyed by a slick wire line or coiled tubing typically incorporate a design including a minimum DC resistance of 50 ohms with a minimum "no fire" current of 200 milliamperes.
Halliburton Energy Services, Inc. provides an electronic triggering device for setting various explosive-powered tools and charges without the use of a conductor line. The device can be run on a mechanical slick line, braided line or sand line, and the tool is used for multi-shot perforating, cutting tubing or casing, setting bridge plugs or packers and other services including dump baler, string shot and patching tubing or casing. The device is battery powered, sensitive to motion, and has two pressure-activated switches. The pressure-activated switches are closed typically at a depth of about 300 feet by fluid pressure. Thus, the pressure-activated switches provide a safety feature to prevent an explosion on or near the surface. After the pressure-activated switches are closed, a motion sensor prevents detonation as long as the motion sensor detects motion.
Although Halliburton's electronic triggering device represents the state of the prior art, it has several deficiencies. This prior art triggering device relies entirely on a motion sensor to prevent detonation after the pressure-activated switches are closed, and a motion sensor does not provide a fully predictable detonation, particularly in a horizontal well. Where run-in time is lengthy, due to possibly the depth of the well or the use of coiled tubing which is run in relatively slowly, an operator must rely on the motion sensor for an extended period of time to prevent detonation. If the prior art triggering device has a timeout circuit for deactivation after a predetermined period of time, then a long run-in time may cause the device to deactivate prematurely or the period of time must be set relatively long, sometimes resulting in unnecessary delay.
Safety is a her concern because the triggering device is sometimes placed inside a lubricator in a well at the surface, where it can encounter sufficient pressure to close the pressure-activated switches which activates the tool. The triggering device then must be lowered before a timer allows it to explode. On information and belief a further deficiency is that the prior art triggering device cannot be controlled from the surface.
Implementation of the recommendations of the American Petroleum Institute in its Recommended Practice 67 created another deficiency. The prior art triggering device uses a large number of batteries to meet the current and voltage requirements to detonate an explosive, so many batteries that the tool is necessarily very long. Although the prior art tool disassembles into sections, the longest section is so long (due to the large number of batteries) that it cannot be positioned in or transported by helicopter, which is a preferred means of transportation for equipment and personnel to offshore oil and gas wells. The long length of the prior at triggering device is also undesirable because it limits the angle that can be drilled between a vertical and horizontal run in a deviated well, since the tool cannot pass through a tight radius bend.